... 2x3=2. (4. 7)2)x1− 2x2+3x3− x 4 =6,2x1+3x2− 4x3+4x 4 =7,3x1+ x2− 2x3− 2x 4 =9,x1− 3x2+7x3+6x 4 = −7. (4. 8)Gia’i. 1)´Ap du.ng cˆong th´u.c (4. 4)xj=det(Aj)detA,j=1, ... 3x 4 =8,3x1+2x2− x3+2x 4 =4, 2x1− 3x2+2x3+ x 4 = −8.(DS. x1=1,x2=2,x3= −1, x 4 = −2)33.x2− 3x3+4x 4 = −5,x1− 2x3+3x 4 = 4, 3x1+2x2− 5x 4 =12,4x1+3x2− ... x1=1,x2=2,x3=1,x 4 = −1) 34. x1+ x2− x3+ x 4 =4, 2x1− x2+3x3− 2x 4 =1,x1− x3+2x 4 =6,3x1− x2+ x3− x 4 =0.(DS. x1=1,x2=2,x3=3,x 4 =4) 35.x1+ x2+ x3+ x 4 =0,x2+...